The following text should not be construed as an admission of knowledge in the prior art. Furthermore, citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention, or that any reference forms a part of the common knowledge in the art.
In the field of prosthetics, many components are operated using power harvested from the user's own musculature using harnessing and a Bowden cable like those used for bicycle brakes and derailleurs. This form of operation is called body-powered, or BP. One such BP component is called a terminal device (TD) or gripper, and its function is to replace a missing hand for upper-limb amputees. It is a device that permits the user to grasp objects. Two configurations of TD are commonly used: voluntary-opening (VO), and voluntary-closing (VC).
With VO operation, the terminal device opens as the user increases their cable tension, due to an applied force. To grasp an object, they first apply a force to open the TD to larger than they require for the object, called sizing, or sizing the aperture, and then they move the TD around the object or bring the object between the grasping digits, hook structures, etc. Relaxing the cable tension allows the device to close and apply a prehension force to the object. This system behaves mechanically identical to a spring-loaded clamp.
Advantages of VO devices are that the TD will grasp an object even if the user allows the cable tension to vanish or go slack. This prevents muscle fatigue. The devices are very simple, and use replaceable rubber bands or springs to generate a prehension force. Drawbacks are that the prehension or pinch force maximum is set by the strength of the rubber bands. Many users add bands to generate a higher force, but then must modulate the cable tension to avoid crushing delicate objects. This requires muscular exertion to offset a portion of the pinch force, causing fatigue and over a period time secondary health consequences that can be debilitating and even devastating. Repetitive stress or overuse syndrome, cumulative traumatic injury, pain, loss of bodily function, and significant medical treatment costs are common, along with lost work.
However, if the user operates the cable such that it is rarely allowed to go slack, the user can develop a sense for how much pinch force they are applying. This ability to sense objects through the cable is called physiological proprioception. It is the same physiological connection that allows a tennis racket, golf club, bat, etc. to become an extension of the user's body; they know precisely where in space around their body the instrument is and are able to use it with extraordinary skill. Cable-operated, or BP terminal devices also preserve proprioception and this is one of the primary reasons they remain popular over more sophisticated technologies.
Voluntary-closing devices actuate in exactly the opposite manner. As the user increases their cable tension, the device closes to apply a prehension force. Advantages are the device preserves proprioception, and pinch force is proportional to the cable tension, due to the applied force, allowing the user to “feel” the object being grasped, and operation is more intuitive to the user. Maximum pinch force is determined by the user's strength, not elastic bands as with VO operation. Drawbacks of VC operation are that at rest, the TD is open, which can be awkward when it is not being used. Also, to sustain prehension, the user must sustain their cable tension, often resulting in fatigue or dropping objects if the user moves their torso and cable tension fluctuates. For general carrying or holding objects passively, VO operation is preferred, while dexterous tasks benefit from VC operation. Each mode has its applications and areas of performance where it excels.
At least two reasons exist that have inhibited the development of a gripping device with a simple design for easily switching between prehension modes. First, in VO mode, the TD is held closed with a strong spring force. In VC mode, it must be held open with a weak or light force to avoid fatiguing the user. Changing positions of the moving digit from open to close while simultaneously changing the spring force is a difficult engineering problem. Second, the device must change operating modes from VO to VC and back without changing the cable resting position. If the cable moves or suddenly gets tensioned or highly slack, the user will be unable to operate the device without having to adjust their harness. This is difficult for an amputee to achieve. Harness and cable settings are exquisitely sensitive to variation and minor changes can render the system unusable. This is one of the chief reasons upper limb prosthetic fitting and adjustment is considered very difficult and an area of prosthetic expertise.
Thus, it is clearly evident that there is a long-felt need in the field of prosthetics for a body-actuated gripping device with an easily switchable prehension mode (from VO mode to VC mode and back again), that is also simple in design and operation, easy to manufacture, and economical from both a purchase price perspective and maintenance perspective.